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Left: The Carlsbad, Calif., seawater desalination demonstration plant is a 30,000 gpd plant, it has a potable water sampling station that allows visitors to taste the desalinated water. Above: The reverse osmosis system consists of two four-element pressure vessels in series. This configuration allows collection of permeate from one or both ends of each vessel and testing of different numbers of membrane elements. Photos: Nikolay Voutchkov
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An ongoing marine aquarium test places aquatic organisms indigenous to power plant discharge areas in water with a salinity equivalent to what would be experienced with the full scale desalination plant in operation. After nine months, the species are healthy and tolerant of the environment. Photo: Nikolay Voutchkov

The demonstration desalination plant intake withdraws warm water from a small discharge pond, which is located at the end of the power plant discharge tunnel. The power plant discharge cooling water is typically 5° to 10° F warmer than the seawater. The intake seawater's total dissolved solids (TDS) concentration varies from 33,000 mg/L to 34,500 mg/L. The demonstration plant's dry-weather intake water turbidity is usually 1 to 4 nephelometric turbidity units (NTU). During wet-weather conditions, which are usually brief and occur mostly in the winter, raw seawater turbidity varies from 6 to 12 NTU, with occasional hourly spikes of up to 24 NTU.

The intake seawater is conveyed to a feed storage tank, where it is then pumped to the demonstration plant pretreatment systems. Currently, the two pretreatment systems undergoing testing are a continuous backwash granular media filtration system and an immersed microfiltration (MF) system.

The granular-media pretreatment system includes two continuous backwash filters in series. The first filter has a coarse (0.9 mm) sand media bed. The second filter contains finer (0.5 mm) sand media. Both filters have instrumentation for continuous turbidity monitoring and data logging. The second-stage filter also is equipped with a particle counter.

The MF system consists of a test vessel, which contains several immersed membrane modules and typically operates under less than 1 to 3 psi of vacuum. This system also is equipped with instrumentation for feed water and filtrate turbidity monitoring and for automated data reporting and acquisition.

The two pretreatment systems are operated independently and typically each produces 40 to 45 gpm of filtered water. The feed water to the granular media pretreatment system is conditioned using ferric sulfate. The MF system does not use chemicals for raw seawater conditioning and is tested at filter effluent production rates similar to those of the granular media system.

The RO system consists of two four-element pressure vessels in series. This configuration allows collection of permeate from one or both ends of each vessel and testing of different numbers of membrane elements. The tested seawater RO membrane elements are 8-inch diameter, high-salt-rejection units. The RO system is designed to run in a range of 45% to 55% recovery and typically operates at 50% recovery. The effects of operation at more and less than 50% recovery on key system parameters such as permeate TDS concentration, feed pressure, and other parameters are planned to be tested over the course of the demonstration study.

Carlsbad's Success

The demonstration desalination plant has been in continuous operation since August 2003. The quality of the produced permeate is consistently high—TDS concentration of 200 to 300 mg/L at RO system feed pressure of 780 to 900 psi. The two pretreatment systems are performing well and the filtered seawater of the systems typically has a silt density index of less than four and turbidity of less than 0.1 NTU.

In the demonstration scale and the planned full-scale desalination plants, the high-salinity concentrate, which is a side product of the desalination process, is blended back into the warm power plant cooling water prior to discharge to the ocean. Blended discharge is environmentally safe, as indicated by chronic and acute whole effluent toxicity testing of a blend of demonstration plant concentrate and power plant cooling water in a ratio corresponding to full-scale, worst-case discharge conditions.

Voutchkov is senior vice president of technical services for Poseidon Resources, Stamford, Conn.

The fish test

An additional confirmation of the plant's success is an ongoing marine aquarium test where aquatic organisms indigenous to the area of the power plant discharge are exposed to a blend of desalination plant concentrate and power plant cooling water discharge in a ratio reflective of the full-scale facility operations. Seawater leaving the Encina power station's cooling system (33,500 mg/L of total dissolved solids, or IDS) is combined in the aquarium with demonstration plant concentrate (67,000 mg/L of IDS) to maintain salinity concentration within the expected range of the combined power plant/desalination plant discharge (35,700 to 36,200 mg/L of IDS). The marine species in the aquarium—such as the barred sand bass, the California halibut, the red sea urchin, and the green abalone—are representative of a diverse and healthy indigenous marine environment and most of them are of economic or recreational fishing significance.

The aquarium test is designed to assess the effect of prolonged exposure of the aquatic biota on the elevated salinity conditions and is conducted by a marine biologist with expertise and extensive knowledge of the aquatic life in the vicinity of the power plant intake and discharge. The marine species have adapted seamlessly and, after more than nine months of continuous exposure to the elevated salinity concentration, are healthy and tolerate the new discharge conditions.